TM 5-818-1 / AFM 88-3, Chap. 7
b. For stiff-fissured clays, diagram (c) of figure
ply, the use of total unit weights in calculating earth
pressures automatically accounts for the loads produced
14-11 applies for any value of N. If soft clays, diagram
by groundwater (fig 14-11). Pressures due to the
(b) applies except when the computed maximum
surcharge load are computed as indicated in previous
pressure falls below the value of the maximum pressure
sections and added to the earth and water pressures.
in diagram (c). In these cases, generally for N < 5 or 6,
e. Each strut is assumed to support an area
diagram (c) is used as a lower limit. There are no design
extending halfway to the adjacent strut (fig 14-11). The
rules for stiff intact clays and for soils characterized by
both c and φ such as sandy clays, clayey sands, or
strut load is obtained by summing the pressure over the
corresponding tributary area. Temperature effects, such
cohesive silts.
as temperature increase or freezing of the retained
c. The upper tier of bracing should always be
material, may significantly increase strut loads.
installed near the top of the cut, although computations
f.
Support is carried to the sheeting between
may indicate that it could be installed at a greater depth.
the struts by horizontal structural members (wales). The
Its location should not exceed 2su below the top of the
wale members should be designed to support a
wall.
uniformly distributed load equal to the maximum
d. Unbalanced water pressures should be
pressure determined from figure 14-11 times the spacing
added to the earth pressures where the water can move
between the wales. The wales may be assumed to be
freely through the soil during the life of the excavation.
simply supported (pinned) at the struts.
Buoyant unit weight is used for the soil below water.
Where undrained behavior of a soil is considered to ap-
Table 14-2. Factors Involved in Choice of a Support System for a Deep Excavation
Lends Itself To Use
Downgrades Utility
Requirement
Of
Of
Comment
1.
Open excavation area
Tiebacks or rakers or
Crosslot struts
cantilever walls (shal-
low excavation)
2.
Low initial cost
Soldier pile or sheetpile
Diaphragm walls, cyl-
Depends somewhat on 3
walls; combined soil
inder pile walls
slope with wall
3.
Use as part of per-
Diaphragm or cylinder
Sheetpile or soldier
Diaphragm wall most com-
manent structure
pile walls
pile walls
mon as permanent
wall
4.
Deep, soft clay sub-
Strutted or raker sup-
Tiebacks, flexible
Tieback capacity not
surface conditions
ported diaphragm or
walls
adequate in soft clays
cylinder pile walls
5.
Dense, gravelly sand
Soldier pile, diaphragm
Sheetpile walls
Sheetpile walls lose in-
or clay subsoils
or cylinder pile
terlock on hard driving
6.
Deep, overconsoli-
Struts, long tiebacks
Short tiebacks
High lateral stresses are
dated clays
or combination tie-
relieved in O.C. soils
backs and struts
and lateral movements
(fig. 14-10)
may be large and
extend deep into soil
7.
Avoid dewatering
Diaphragm walls, pos-
Soldier pile wall
Soldier pile wall is
sibly sheetpile walls
pervious
in soft subsoils
8.
Minimize movements
High preloads on stiff
Flexible walls
Analyze for stability of
strutted or tied-back
bottom of excavation
wall
9.
Wide excavation
Tiebacks or rakers
Crosslot struts
Tiebacks preferable ex-
(greater than 20m
cept in very soft clay
wide)
subsoils
10. Narrow excava-
Crosslot struts
Tiebacks or rakers
Struts more economical
tion (less than
but tiebacks still may
20m wide)
be preferred to keep
excavation open
U. S. Army Corps of Engineers
14-14
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